1. Field of the Invention
The present invention relates to an apparatus and a method for bottom casting of a molten steel.
2. Description of the Related Art
As compared with a top casting method, a bottom casting method can provide higher efficiency in casting a molten steel and eliminate possible splashing of a molten steel in a mold, causing a smooth surface of an ingot. Therefore, the bottom casting method is adapted to cast a steel such as a stainless steel.
An apparatus for use in bottom casting, comprises a stool, a pouring pipe and a mold mounted thereon. In the apparatus, a bottom surface of the pouring pipe communicates with a bottom surface of the mold through a runner brick disposed within a groove formed in the stool, allowing molten steel to flow through the pouring pipe into the mold. The pouring pipe generally comprises a cylindrical outer barrel made of cast iron and a cylindrical pouring pipe brick disposed within the outer barrel.
In casting performed by the bottom casting method, molten steel poured into the pouring pipe flows through the runner brick into the mold, and solidifies to form an ingot within the mold. While molten steel within the mold is cooled, molten steel remaining within the pouring pipe and the runner brick also solidifies. Therefore, in each ingot casting completion, the pouring pipe and the runner brick which have been used are removed and replaced with fresh ones.
A work in which the runner brick is disposed in the stool is called "runner brick setting". The work is conventionally performed in the following manner. That is, after the bottom of the groove formed on the stool is filled with sand, a plurality of runner bricks are disposed within the groove along the length thereof, and seam between adjacent runner bricks is sealed. Thereafter, a space between a side wall of the groove and the runner bricks is filled with sand, and an upper portion of sand is covered with mortar.
However, in the conventional method mentioned above, a work for filling the sand and a work drying the mortar securing the sand are required, thereby lowering workability and polluting the working ambiance due to occurrence of dust. Accordingly, for solving the above problem, methods for setting runner bricks without using sand and/or mortar have been disclosed in Japanese Patent Application Publication (JP-B) No. 53-35897, and Japanese Patent Application Laid-Open (JP-A) No. 6-218494.
In accordance with the method which has been disclosed in JP-B No. 53-35897, the width of each lateral gap between the groove and the runner brick is set to 6 mm or less, or metal plate is inserted in each gap in respective sides to adjust the width of the gap to be 6 mm or less. Even when molten steel flows out due to accidental cracking of the runner brick, the molten steel is cooled and solidified by the side wall of the groove of the stool or the inserted metal plate. Accordingly, molten steel can be prevented from flowing out further. However, in this method, when the width of the gap between the groove and the runner brick is, for example, about 4 mm, the opening width of the crack also become about 4 mm. Therefore, flowing-out of molten steel can not be prevented perfectly.
The method which has been disclosed in JP-A No. 6-218494 has been proposed by the assignee of the present invention so as to overcome disadvantages of the method which has been disclosed in JP-B No. 53-35897 mentioned above. In this method, the width of the lateral gap between the groove and the runner brick, and a difference between the depth of the groove and the height of the runner brick, which are measured in thermal expansion of the runner brick, are made 0.5 mm or less. Even when a crack accidentally occurs in the runner brick, an opening width of the crack can be suppressed to 0.5 mm or less, thereby providing molten steel from flowing out.
As mentioned above, after one casting work is completed, the used runner bricks are removed from the stool. The simplest way for removing the runner bricks one where, after the ingot, the outer barrel of the pouring pipe and the mold are removed from the stool, cause the stool is lifted up with a crane and reversed to cause the runner bricks and the pouring bricks to fall off at a stroke together with solidified material remained therein.
When this removing technique is applied to the method which has been disclosed in the JP-A No. 6-218494, the runner bricks on an upper region of the groove breaks and falls with the solidified material remaining in the runner, allowing them to be removed. However, since broken pieces or chips originated the runner bricks enter into a gap between the runner bricks disposed at a bottom side of the groove and the groove, some of the pieces of the runner bricks remain in the groove in some cases. In this case, it is necessary to remove the remaining pieces from the groove. For example, the reversed stool is returned back to its original state and a removing work must be performed using a breaker or the like. Accordingly, it takes a long time for a post-processing (post-treatment) after ingot casting.